The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Hydrocarbons (oil, condensate, and gas) are typically produced from wells that are drilled into the formations containing them. For a variety of reasons, such as inherently low permeability of the reservoirs or damage to the formation caused by drilling and completion of the well, the flow of hydrocarbons into the well is undesirably low. In this case, the well is “stimulated” for example using hydraulic fracturing, chemical (usually acid) stimulation, or a combination of the two (called acid fracturing or fracture acidizing).
In hydraulic and acid fracturing, a first, viscous fluid called the pad is typically injected into the formation to initiate and propagate the fracture. This is followed by a second fluid that contains a proppant to keep the fracture open after the pumping pressure is released. Granular proppant materials may include sand, ceramic beads, or other materials. These types of materials are well known to those skilled in the art. In “acid” fracturing, the second fluid contains an acid or other chemical such as a chelating agent that can dissolve part of the rock, causing irregular etching of the fracture face and removal of some of the mineral matter, resulting in the fracture not completely closing when the pumping is stopped. Occasionally, hydraulic fracturing can be done without a highly viscosified fluid (i.e., slick water) to minimize the damage caused by polymers or the cost of other viscosifiers.
In gravel packing, gravel is placed in the annulus of screen and formation/casing to control formation sand production. A carrier fluid is used to transport gravel from the surface to the formation where the gravel has to be placed. Typically two types of carrier fluids are used. The first is a brine with a low concentration of gravel (1 lb per gal of brine) and the second is a viscous fluid with higher concentration of gravel (5 lb per gal of brine). Several types of viscosifiers are used to increase the viscosity of the fluid. These include polymers such as HEC, Xanthan, Guar and guar derivatives, etc. and viscoelastic surfactants. In a combination of gravel packing and hydraulic fracturing, referred to in the industry as frac and pack, a viscous fluid is typically pumped to create/propagate a hydraulic fracture by pumping a pad fluid followed by a slurry (fluid containing proppant) to completely fill the hydraulic fracture. In this type of treatment it is critical to completely fill the fracture to the point where no additional propagation occurs and the remaining slurry is “squeezed” into the hydraulic fracture with increasing pressure. This is referred to in the industry as a tip screenout (TSO). This is important because the next step of the frac and pack is to shift the flow of the fluid being pumped to now perform the gravel pack part of the operation. This technique is known by those skilled in the art.
Conventionally, the components are mixed at the surface prior to being pumped into the well with a pumping device. Further peripheral devices are present such as a blender, a particulates hauler, fluid storage tank(s), and other devices understood in the art. Water from the storage tank is mixed with a viscosifying agent or various additives to create the base fracturing fluid. Proppant or particulates are added to the fracturing fluid from a conveyor at the blender, or are added directly by the blender (this method is on-the-fly). In some gravel pack operations, and may be done rarely in frac and pack treatments, the base fluid and particulates (proppant) are batch-mixed before being pumped in the wellbore.
The mixing of the components has to be done on site and prior to pumping into the well, because the transport and storage of already prepared solid fluids (with proppant, gravel, or other particulate material) suffer from a lot of drawbacks. A common problem that occurs during pre-mixed slurry transport or storage is the setting of solids due to difference in densities of the fluid and the solid particles. If the solids start settling before the fluid is used, the fluid will need to be remixed prior to use. If use of such settled fluid was tried, several problems can occur including screen outs, incomplete gravel packs, wellbore blockage, stuck tools etc. To reduce the settling rate, the carrier fluid is typically viscosified using polymers or surfactants. However, increasing the viscosity of the fluid can increase the friction pressure significantly and renders the fluid not pumpable. An alternative would be to add fiber to the fluid mixture (FiberFRAC—Mark of Schlumberger). In this case the fluid viscosity essentially stays the same but the proppant suspension is significantly better. Other methods could be to use lighter density proppant so that the density between fluid and proppant is minimized so that proppant suspension is improved. The best case would be where the fluid and the proppant are equal density.
Compositions and methods disclosed herewith offer a new way to deliver fluid with variable solid concentrations on well site.